This is a cheat sheet for working with lists in Python. It’s an example of all items list in Essential Interview Code for Lists and Vectors. Knowing this, or having the list handy, will speed you along in coding interviews.
Basics
# Create a list mylist = [] # Add an element to the front or back mylist.append( "abc" ) mylist.insert(0, "def") # Pop element off front or back end = mylist.pop() start = mylist.pop(0) # Forward iterate over elements for item in mylist: print( item ) # Get the length of list len( mylist ) # Test if empty if not mylist: print( "list is empty" ) ## Location Based Operations # Get item at location mylist[2] # Insert an item at location mylist.insert( 3, "abc" ) # Remove an item from location del mylist[2] # Replace/Assign item at location mylist[1] = "def"
Sorting and searching
# Find an item if item in mylist: index = mylist.index(item) # Using `index` and error handling try: index = mylist.index( 'abc' ) except ValueError: index = None # Using `next` and filtering next((x for x in mylist if x == 'ghif'), None) # Find and remove an item if item in mylist: mylist.remove(item) # with error handling try: mylist.remove( item ) except ValueError: pass # Find last matching item # Index of found item, or None next( (index for index in reversed(range(len(mylist))) if mylist[index] == item), None) # Alternately, reverse list and use "Find an item", but that copies the list revlist = mylist[::-1] if item in revlist: index = revlist.index(item) # Sort by natural order # in-place sort mylist.sort() # Sort with custom comparator mylist = [ ('a', 10), ('b', 7), ('c',13), ('d',1) ] # sort by a key (sub-element mylist.sort( key = lambda item: item[1] ) # custom comparator def compare_fn( a, b ): return some_cond(a,b) mylist.sort( key = functools.cmp_to_key( compare_fn ) )
Segment Manipulation
# Split the list at arbitrary location tail_of_list = mylist[2:] head_of_list = mylist[:2] # Multiple splits based on a match mylist = ['a', 'b', 'c', 'd', 'b', 'e'] [list(y) for x, y in itertools.groupby(mylist, lambda z: z == 'b') if not x] # Clear the list mylist.clear() # Remove segment # delete from position 1 up to, but excluding position 3 del mylist[1:3] # Concatenate lists mylist + other_list # Insert list at location # list slicing replaces the segment of list with another one, here we replace a zero-length slice mylist[1:1] = other_list # Get a sublist # sublist starting at position 1 up to, but excluding, position 3 mylist[1:3]
More Iteration
# Backward for item in reversed(mylist): print(item) # Partial segment iteration # using itertools.islice avoids copying the list (which is what would happen if you used a slice) for item in itertools.islice( mylist, 1, 4 ): print(item) # Skipping elements # step from element 1 to 6 (exclusive) by 2 for item in itertools.islice( mylist, 1, 6, 2 ): print(item)
Creation
# Create from a static list of items mylist = [ 'abc', 'def', 'ghi'] # Create a range of numbers # a list of numbers from 10..20 (exclusive) numbers = list(range(10,20))
Data Manipulation
# Mapping [number * 10 for number in numbers] # Filtering [number for number in numbers if number % 2 == 0] # Fold / Reduce # Summing up numbers using builtin add functools.reduce( operator.add, numbers ) # Joining string representations of items functools.reduce( lambda left,right: str(left) + '/' + str(right), mylist ) # Zip # the zip function produces a list of tuples zip( lista, listb ) # to alternate items into one list use reduce functools.reduce( operator.add, zip(lista, listb) )
Advanced
# Swap elements at two locations mylist[3], mylist[5] = mylist[5], mylist[3] # Reserve capacity # Python lists do not expose capacity # Replace content in a list mylist[:] = other_list # Compare two lists lista == listb # Search a sorted list # bisect_left/bisect_right work with sorted lists, # find an item ndx using bisect_left, finds the left-most item ndx = bisect_left(numbers, 4) if ndx != len(numbers) and numbers[ndx] == 4 print( "Found at {}".format(ndx) ) # Iterators # Manually stepping through an iterator myiter = iter(mylist) while True: try: n = next(myiter) print(n) except StopIteration: break # Multiple iterators at the same time itera = iter(lista) iterb = iter(listb) while True: try: a = next(itera) b = next(iterb) print(a,b) except StopIteration: break
